The invention relates to relatively short peptides (termed .alpha.-conotoxins herein), about 10-25 residues in length, which are naturally available in minute amounts in the venom of the cone snails or analogous to the naturally available peptides, and which preferably include two disulfide bonds. The .alpha.-conotoxins, as described herein, are useful for as neuromuscular blocking agents, such as muscle relaxants.
The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference, and for convenience are referenced in the following text by author and date and are listed alphabetically by author in the appended bibliography.
The predatory cone snails (Conus) have developed a unique biological strategy. Their venom contains relatively small peptides that are targeted to various neuromuscular receptors and may be equivalent in their pharmacological diversity to the alkaloids of plants or secondary metabolites of microorganisms. Many of these peptides are among the smallest nucleic acid-encoded translation products having defined conformations, and as such, they are somewhat unusual. Peptides in this size range normally equilibrate among many conformations. Proteins having a fixed conformation are generally much larger.
The cone snails that produce these peptides are a large genus of venomous gastropods comprising approximately 500 species. All cone snail species are predators that inject venom to capture prey, and the spectrum of animals that the genus as a whole can envenomate is broad. A wide variety of hunting strategies are used, however, every Conus species uses fundamentally the same basic pattern of envenomation.
Several peptides isolated from Conus venoms have been characterized. These include the .alpha.-, .mu.- and .omega.-conotoxins which target nicotinic acetylcholine receptors, muscle sodium channels, and neuronal calcium channels, respectively (Olivera et al., 1985). Conopressins, which are vasopressin analogs, have also been identified (Cruz et al. 1987). In addition, peptides named conantokins have been isolated from Conus geographus and Conus tulipa (Mena et al., 1990; Haack et al., 1990).
The .alpha.-conotoxins are small peptides highly specific for neuromuscular junction nicotinic acetylcholine receptors (Gray et al., 1981; Marshall and Harvey, 1990; Blount et al., 1992). The .alpha.-conotoxin peptides MI and GI are selective for the .alpha./.delta. subunit interface of the neuromuscular junction nicotinic receptor over the .alpha./.gamma. subunit interface by &gt;10,000 fold, while the .alpha.-conotoxin peptides EI and EIA bind both sites with equal affinity. However, none of these peptides show significant affinity for neuronal nicotinic receptors.
Various compounds having muscle relaxant properties are set forth in U.S. Pat. Nos. 4,190,674; 4,508,715; 4,761,418; 4,701,460; 4,179,507; 4,923,898; 5,015,741; and 5,260,337; as well as in Goodman and Gilman's The Pharmacological Basis of Therapeutics, Section II, especially Chapter 11, 7th Ed. (1985) and Physicians Desk Reference, 48 Ed., pp. 689, 758, 1362 and 1648 (1994).
Compounds having musculoskeletal relaxing properties include (1) agents acting in the central nervous system which are used to relieve pain associated with muscle contraction (e.g., 5-chlorobenzoxazolinone available as Parafon Forte DSC from McNeil Pharmaceutical), and (2) agents acting in the peripheral nervous system used primarily to induce muscle relaxation and hence reduce muscle contraction during anesthesia. The second group of muscle relaxants is subdivided into two groups: (i) non-depolarizing agents which inhibit the activation of muscle receptors (e.g., metocurarine iodide, d-tubocurarine, tubocurarine chloride, pancuronium, gallamine, diallytoiferine, toxiferine, atracurium besylate which is available as Tracrium from Burroughs-Wellcome Co., and vecuronium bromide which is available as Norcuron from Organon Inc.) and (ii) depolarizing agents which transiently activate muscle receptors and result in their blockade (e.g., decamethonium iodide, and succinylcholine chloride which is available as Anectine from Burroughs-Wellcome Co.). The effects of the depolarizing agents are manifested as fasciculations and flaccid paralysis which are observed to occur rapidly after their injection.
The effects of depolarizing agents (DA) and non-depolarizing agents (NDA) are separated based on their duration of action from ultrashort acting (e.g., for a depolarizing agent such as succinylcholine chloride) to intermediate (e.g., for a non-depolarizing agent such as atracurium besylate). Certain types of muscle relaxants are useful as neuromuscular blocking agents in clinical applications, and have found use as adjuvants to surgical anesthesia, in orthopedic surgical procedures and in facilitating endotracheal intubation procedures. Some of these compounds (e.g., succinylcholine chloride) are routinely used to provide muscle relaxation during Cesarean section procedures.
It is desirable for neuromuscular blocking agents to be locally acting and highly selective for binding to muscle nicotinic acetylcholine receptor sites. As such, when a patient is treated with anesthesia, the muscle relaxant is applied (e.g., intravenously or by injection), in order to cause the muscle to relax and hence minimize muscle contraction.
In anesthesia, neuromuscular blocking agents are used to provide skeletal muscular relaxation during surgery and during intubation of the trachea. All of the conventional nondepolarizing agents when used for producing skeletal muscle relaxation in surgery have a long duration of action e.g., 60 to 180 minutes in man. The depolarizing agents on the other hand provide muscle relaxation at dosages normally used for surgery which is less than the duration of action of nondepolarizing agents. For example, succinylcholine provides a short duration of action of about 5 to 15 minutes whereas decamethonium provides about 20 to 40 minutes duration of muscle relaxation. The long duration of action of nondepolarizing agents is unacceptable in many surgical procedures which take less than one hour because the patient is not generally fully recovered from their effects e.g., the patient may be unable to breathe adequately on his or her own.
Each nondepolarizing agent has inherent side-effects. For example, gallamine and pancuronium may cause tachycardia, d-tubocurarine and diallyltoxiferine may cause hypotension, and succinylcholine may cause fasciculations, myalgia, potassium release, cardiovascular effects, immunological reactions and malignant hyperthermia. While such drugs can be pharmacologically antagonized with anticholinesterase agents, this obviously necessitates the administration of a second drug which itself may have its own side effects e.g., bradycardia, gut spasm and bronchorrhea. Thus to overcome the aforementioned side-effects of the anticholinesterase agents, a third drug, an anticholinergic drug e.g., atropine must also be given.
With the use of depolarizing agents, there is no need to reverse the effects of the depolarizing agents, in certain patients the effects are much prolonged because of abnormal metabolism of the agent by the patient. The polarizing agents due to the mode of action which initially causes skeletal muscle contraction and stimulation of smooth muscles are also known to cause the following side-effects in certain instances; increased intraocular, and intragastric tension, cardiac arrhythmias, potassium release, and muscle pain. These side-effects caused by the depolarizing agents are not caused by the nondepolarizing agents. It is therefore clearly evident that a new neuromuscular blocking agent having the relatively few side-effects and the reversibility of the nondepolarizing agents yet being of considerably shorter i.e., intermediate, duration of action is needed.
It is desired to provide a compound useful as a muscle relaxant. In particular, it is desired to provide an agonist which has activity at relatively low concentrations as a neuromuscular blocking agent. It is also desired to achieve muscle relaxation at concentrations of agonist that are devoid of any ganglionic effects (e.g., so as to not exhibit side effects such as those associated with interaction with cardiovascular sites). As such, it is desired to provide muscle relaxant compositions and methods for providing muscle relaxation. Finally, it is desired to identify additional .alpha.-conotoxin peptides for use as neuromuscular blocking agents.